The present invention relates to separators for electric storage batteries, where there is risk of bridge formation between the electrodes by non-insulating material. This ordinarily results from formation of dendrites by the electrodes.
In an electric storage battery there are two different kinds of electrodes in each cell, positive and negative. There must not be any electrical contact between these electrodes other than by way of the electrolyte in each cell. However, since in many kinds of electrodes, there is a tendency to dendrite formation, i.e., outgrowths of electrode material from the electrode, the electrodes must be kept separated in some way, so that these dendrites will not bring about direct electrical conductive contact between the electrodes. Such undesirable contact can be prevented, of course, by placing the electrodes far apart. However, there is then very great electrical resistivity in the cells, and the structure is also bulky. It is therefore necessary to solve the problem in some other way. It was decided to use separators. These are porous sheets of various materials such as wood, paper and plastic. In order that the separators may tolerate the milieu of the battery cell, they have to be made either of material with acceptable strength, for instance, polyvinyl chloride, or else suitably treated, e.g., phenolic resin impregnation of paper. Depending upon the way in which the separator is made, there are different pore sizes, and usually the distinction is made between microporous and macroporous separators, the borderline being drawn at an average pore size of about 10.mu..
Microporous battery separators are known in the art and have been produced in different ways, using various materials to form a self-supporting film or matrix. Representative products and methods for making them are disclosed, for example, in U.S. Pat. Nos. 2,537,631; 2,700,694; 2,772,322; 2,862,988; and 2,913,512.
In the manufacture of such separators, there is always the risk that there will be pinholes in them. By pinholes, we mean holes with a diameter larger than the sought pore size. Pinholes are also characterized in that they establish direct linear connection between the two sides of the separator. Because of the great risk of short circuits from such a hole, all separators have to be closely inspected after manufacture, and separators with pinholes have to be rejected. In spite of close checking in separator manufacture, it may happen that a separator with pinholes will be used in manufacture of a storage battery cell. The risks of short circuit and consequent malfunction or failure of the cell are then very great. Since, however, the separators increase the internal electric resistivity of the cell, the effort is made to produce separators that are as thin as possible. Thin separators offer additional advantages, such as less bulky construction, and the possibility of better utilization of the available space by filling it with active material. However, the thinner the separator, the greater the risk of formation of pinholes, and separators have therefore generally been made with a minimum thickness of 0.5 mm, generally between 0.5 and 1 mm.
Accordingly, a prime object of the present invention is to produce microporous separators with improved security against internal battery short circuit resulting from pinholes as herein defined.
Another object is to provide a separator which permits the use of compositions previously considered undersirable or dangerous because of the risk of pinholes. Still another object is to provide a battery separator affording security against short circuit due to pinholes while exhibiting low electrical resistance.
These and other objects, which will be apparent to those skilled in the art, are achieved in accordance with the present invention.